1. Field of the Invention
The present invention relates to a light-absorbing layer for a solar cell, and more particularly, to a chalcopyrite light-absorbing layer for a solar cell.
2. Description of the Related Art
Recently, the development of next-generation clean energy is regarded as important because of serious environmental pollution problems and the exhaustion of fossil energy. Since solar cells, which are a device for directly converting solar power into electric energy, generate less pollution, tap an unlimited resource, and have an almost unlimited lifetime, they are expected to be an energy source that is able to solve the energy problems in future.
Solar cells are variously classified depending on the type of material used for the light-absorbing layer. At present, the most widely used is a silicon (Si) solar cell. However, as the price of Si is drastically increasing due to the shortage thereof, solar cells that use compound semiconductors are receiving attention. Compound semiconductors are particularly useful for manufacturing thin film-type solar cells, can be used in small amounts, and are lightweight, and thus are widely applicable.
Generally, a Group I-III-VI2 chalcopyrite compound semiconductor, for example, CuInSe2, has a direct transition-type energy band gap, and possesses a light absorption coefficient of 1×105 cm−1, which is the highest among semiconductors, thus making it possible to manufacture a high-efficiency solar cell using a thin film having a thickness of 1 to 2 μm and exhibiting outstanding long-term electro-optical stability.
Hence, the chalcopyrite compound semiconductor is receiving attention as an inexpensive high-efficiency solar cell material, which may drastically improve the profitability of photovoltaic power generation, in lieu of currently available crystalline Si, which is expensive.
In order to adjust the band gap of CuInSe2 from 1.04 eV to within the range from 1.2 to 1.4 eV, which is ideal, a portion of indium (In) is substituted with gallium (Ga) and a portion of selenium (Se) is substituted with sulfur (S). For reference, the band gaps of CuGaSe2 and CuGaS2 are 1.6 eV and 2.5 eV, respectively.
The quinary compound, in which a portion of In is substituted with Ga and a portion of Se is substituted with S, is represented by CIGSS [Cu(InxGa1-x) (SeyS1-y)2], and may be representatively expressed as CIS or CIGS.
A CIGS light-absorbing layer may be formed using a co-evaporation process. The co-evaporation process is performed by simultaneously evaporating unit elements, for example, copper (Cu), indium (In), gallium (Ga) and selenium (Se), using heat evaporation sources, so that a CIGS thin film is directly formed on a substrate at high temperature. Since individual evaporation sources are independently used, it is easy to control the elemental composition, making it possible to form a CIGS light-absorbing layer having excellent performance. The CIGS solar cells that exhibit the greatest efficiency to date are manufactured through the above process.
In addition, a thin film composed of a precursor material for a CIGS thin film may be formed using sputtering or another deposition process, followed by heat treatment or selenization in a Se or H2Se gas atmosphere.
With the goal of solving the problems of the aforementioned methods, which require an expensive vacuum process, thorough research is ongoing into a non-vacuum process, in which a slurry or ink containing a CIGS precursor material or CIGS compound nanoparticles is prepared, applied on a substrate and then thermally treated.
These days, CZTS (Cu2ZnSn(Se1-xSx)4) solar cells, in which In and Ga of a CIGS light-absorbing layer are substituted with Zn and Sn and which are thus composed of Group I-II-IV-VI elements, which are favorable in terms of material costs and environmental impact, are under active study. The process of manufacturing a CZTS light-absorbing layer for a solar cell is almost the same as the process of manufacturing the CIGS light-absorbing layer. Below, the CIS compound, the CIGS compound and the CZTS compound are defined together as a chalcopyrite compound.
Although heating or heat treatment is required in all of the aforementioned methods of manufacturing the chalcopyrite light-absorbing layer, it is disadvantageous because a substrate, especially a flexible substrate, may be damaged due to such heat.
Recently, chalcopyrite solar cells using molybdenum (Mo) as rear electrodes have become predominant. However, when a chalcopyrite light-absorbing layer is formed on the Mo rear electrode disposed on the substrate, MoSe2 may be excessively produced at the interface between the Mo rear electrode and the light-absorbing layer, attributable to excessive heat treatment, undesirably deteriorating the efficiency of solar cells.